This invention relates generally to TVRO receivers for the reception of a wide range of satellite TV signals and, more particularly, to filtering means for reducing terrestrial interference (TI) and other video noise in a TVRO receiver system.
In a TVRO system, the satellite signals are received by an antenna (usually a paraboloidal dish) and converted to a lower "1st IF" frequency at the antenna site. This conversion may be effected by a down converter, which converts only a single channel to the 1st IF frequency, or a block converter, which converts all channels of a common polarity to a 1st IF block of frequencies ranging from 950 to 1450 MHz. This entire block of frequencies is then fed via coaxial cable to the receiver, which selects a particular channel for viewing and/or listening. In the receiver, the 1st IF signals are converted to a 2nd IF frequency range which traditionally has been centered at 70 MHz in most TVRO systems.
In the reception of TV signals broadcast by satellites, a major problem faced by conventional TVRO receivers is distortion due to terrestrial interference (TI) caused by the presence of local terrestrial microwave communication links. TI can lead to substantial degradation and even total loss of the signals received from a satellite. Previous approaches to this problem have involved the use of various filters within the TVRO receivers to perform wavetrapping within the interference region. The use of such filters is described, for example, in Battle et al., "How to Identify and Eliminate Terrestrial Interference", TVRO Technology, May, 1985, pp. 32-41.
The provision of TI filters is not a major challenge in TVRO receivers operating at 70 MHz or similarly low 2nd IF frequencies. However, the presently emerging generation of TVRO receivers use a higher 2nd IF, e.g., 612 MHz, so that the VCO frequencies are above the 1st IF block of frequencies to prevent the VCO from interfering with the desired signals. At these higher 2nd IF frequencies, it is extremely difficult to produce trap or notch filters with the high Q required to eliminate TI and other video noise without excessive losses in the desired video signal.